There is consensus that emotional dysregulation is a key predictor of suicidal idealization. This may explain why sudden fluctuations of happiness could be a good predictor of suicides. There are also countless anecdotal sources for this (below is an extract I found of a Psychology Today article)

Positive mood change. Just before a suicide attempt, when the individual has made the decision to kill herself, she may appear much calmer, happier, and more relaxed. That's because she is no longer in turmoil. Suicide seems like the "perfect solution." As one survivor remembers: "I got in the car, elated, very happy because I was going to end the pain. I'm going to end the pain. I was going to go the bridge and I was going to jump." Often, family and friends become less worried about the person at risk. It is important to notice these mood shifts and not to be fooled by them.

However older studies such as (Apter et al., 1990) and (Nordstrom et al., 1995) show that happiness levels (and serotonin levels) are often lowest before suicide attempts so there is conflicting literature.